SEASONAL DYNAMICS OF NUTRIENTS IN LEAVES AND XYLEM SAP OF COFFEE PLANTS AS RELATED TO DIFFERENT SOIL COMPARTMENTS

The northwestern province of Costa Rica is a marginal coffee growing a rea. At the onset of the rainy season low redox potentials probably in duce the mobilization of soil Mn resulting in enhanced plant uptake of Mn. To test this hypothesis we monitored from April to the end of Jun e 1995 the mobile Mn in the soil and nutrient and Mn concentrations in leaves and xylem sap of coffee plants. Every 2 weeks we took aggregat e and bulk soil samples. The aggregates were mechanically separated in to interior and exterior, air-dried and all soil samples were extracte d with 1 M NH(4)NO3. We also extracted the field moist soil with disti lled water. In addition, the 3rd and the youngest pair of coffee leave s and xylem sap were sampled and analyzed. According to the results of leaf analyses the nutrient supply of the coffee plants in general see med to be balanced. However, Mn concentrations of 223 mg kg(-1) in the 3rd leaf pair at 18 April were above the optimum and the youngest lea ves indicated Fe deficiency, but senescent leaves accumulated Fe and o vercame the deficiency. Manganese concentrations in the xylem sap show ed a pronounced maximum 2 weeks prior to a similar maximum of mobile M n in the aggregate exterior. But in general the temporal variation of nutrient concentrations (especially Ca and Mg) in the plants are well correlated with the easily extractable nutrient concentrations in bulk soil. Probably due to its specific absorption and high rates of redis tribution within the plant, K in the soil extracts did not correlate w ith plant concentrations. Element concentrations of youngest leaves co uld not be correlated with soil concentrations and are not considered to be an adequate tool for monitoring current nutrient uptake. Since p lant element concentrations did not correlate with the aggregate inter ior, plants probably cannot use that nutrient source efficiently.